Modern elevator systems have doors to permit transfer of passengers between the elevator cars and the respective floor landings. Because smaller doors have to travel a lesser distance and have less inertia, many elevators have two doors. They may meet in the middle, and thereby have a lesser distance to travel or they may both travel to the same side for opening. Other elevators may have only a single door. As used herein, the term "door" or "doors" may be used interchangeably, it being understood that there is no distinction between a single door and double doors concerning the subject matter hereof.
Present day elevator systems have doors mounted on the elevator car, and doors mounted at each hall landing of the elevator hoistway. The hoistway doors at the hall landings are mounted directly to the building structure, and are kept closed whenever the car is not present at the related landing in order to prevent passengers and objects from entering the hoistway. Instead of having door operators for each of the hoistway doors, the hoistway doors are typically opened by coupling them with the car doors, so that opening of the car doors will open the landing doors in unison therewith, thereby protecting passengers in the car from the building structure and protecting passengers at the landing from the hoistway.
For safety reasons, and to comply with safety codes, whenever an elevator car is not adjacent to a landing, the hoistway doors must be safely locked in the closed position. One known type of door lock has a rotatable latch that engages a lip, the latch being rotated by a vane on the adjacent elevator car door, as the car door begins to move toward the open position. This type of lock, however, is useful only in a lost motion door coupling system (that is one in which the car door begins to move before it moves the hoistway door). For systems in which lost motion car door coupling is not permitted, because the resulting perturbations in the door operating system are to be avoided, the vane rotated latch cannot be used. Any mechanisms related to the elevator doors must not interfere with the passage of an elevator past the landing at relatively high speed.
A type of hoistway door lock which can unlock the hoistway door before elevator car door motion occurs utilizes a retiring cam. The retiring cam is mounted on the elevator car and is forced toward the hoistway door as soon as the car approaches the landing sufficiently close so as to begin advance door opening. The retiring cam rotates a member mounted on the hoistway door to unlock the hoistway door prior to any motion of the car door. However, such systems typically lock only one of the hoistway doors directly to the hoistway header or other building structure, and cause the other hoistway door to be locked thereto by a related mechanism of some form. Other devices of this type may require use of a very long rotating member which introduces installation and maintenance problems due to the requirement for critical alignment.